With the recent spread of the Internet, portable phones, digital broadcast and the like, digital communication that transmits and receives multimedia contents such as images and audio are widely available. Since the multimedia contents consists of information being large in amount, a technique to reduce this information amount by encoding (compression encoding) the contents has been commonly used, in order to minimize a transmission bit rate. For example, MPEG (Moving Picture Experts Group)-2 and H.264 have been commonly used as an encoding system for video images.
Such an encoding method employs lossy encoding in which encoding is performed by use of human visual and auditory senses and information difficult for a human to recognize is deleted. Hence, a decoded signal thereof would not be a complete reproduction of an original signal prior to the encoding. Accordingly, the images and sounds indicated by the decoded signal become slightly deteriorated than the images and sounds of the original signal prior to the encoding.
There has been known techniques, which minimize deterioration of a decoded signal while maintaining the degree that the information amount is reduced by the encoding. For example, Patent Literature 1 discloses an encoding technique that optimizes image quality while maintaining a low bit rate, by having quantization be made great in a sector of a video frame in which noise and the like is not easily visualized by a human, whereas having the quantization be made small in a sector of a video frame in which noise and the like can easily be visualized by a human.